Field of the Invention
Embodiments of the present invention relate to variable nozzle turbochargers.
Description of the Related Art
A variable nozzle turbocharger is equipped with a variable nozzle mechanism. A typical variable nozzle mechanism includes variable nozzles having nozzle vanes and a unison ring. The variable nozzle mechanism adjusts the opening degree of the variable nozzles based on a rotation of the unison ring. Thus, the variable nozzle mechanism controls a flow velocity of exhaust gas to a turbine wheel. The unison ring is provided with a drive arm fit-engagement groove that extends radially. A drive arm for driving the unison ring has a fit-engagement portion that is engaged with the fit-engagement groove. The fit-engagement portion is rotatable, and is movable in the radial direction of the unison ring along the fit-engagement groove of the unison ring. Unison-ring/drive-arm engagement structures according to related-art examples 1 and 2 will be described with reference to FIGS. 8 and 9.
As shown in FIG. 8, a drive arm 1 of related-art example 1 has a first end (base end) and a second end (tip end). The first end is rotated around a pivot 2. The second end has a round fit-engagement portion 3. A fit-engagement groove 6 is formed in a unison ring 5 so as to cross it radially and straight. A closing side surface 6a is situated on the side of the fit-engagement groove 6 where the unison ring 5 decreases the opening degree of the variable nozzle. An opening side surface 6b is situated on the side of the fit-engagement groove 6 where the unison ring 5 increases the opening degree of the variable nozzle. The wall surfaces 6a and 6b are flat surfaces facing each other in parallel with a fixed groove width 6W therebetween.
As shown in FIG. 9, related-art example 2 has a fit-engagement groove 8 instead of the fit-engagement groove 6 of FIG. 8. The fit-engagement groove 8 has a closing side surface 8a and an opening side surface 8b. Japanese Laid-Open Utility Model Publication No. 61-49002 discloses a substantially semi-circular fit-engagement groove instead of the fit-engagement grooves 6 and 8. The pressure of exhaust gas, i.e., the so-called exhaust reaction force, acts on the nozzle vane. The exhaust reaction force is generally constantly generated from the variable nozzle side to the actuator side. Thus, the fit-engagement portion 3 of the drive arm 1 constantly contacts the closing side surface 6a or 8a of the fit-engagement groove 6 or 8.
In related-art example 2 of FIG. 9, the fit-engagement portion 3 and the wall surface 8a contact each other. Thus, an arcuate surface contacts another arcuate surface. On the other hand, in related-art example of FIG. 8, the fit-engagement portion 3 and the wall surface 6a contact each other. Thus, an arcuate surface contacts a flat surface. As compared with related-art example 2, in related-art example 1, the contact area is smaller, and the contact stress is larger. As a result, the wall surface 6a of related-art example 1 is more subject to wear than the wall surface 8a of related-art example 2.
In related-art example 2 of FIG. 9, arcuate surfaces contact each other. As compared with related-art example 1 of FIG. 8, the contact stress is reduced. As a result, the wear of the wall surface 8a of related-art example 2 is reduced. However, it is impossible to form simultaneously on both wall surfaces 8a and 8b by using a rotary tool such as an end mill. Thus, it is necessary to form on the wall surfaces 8a and 8b separately. The groove width of the fit-engagement hole 8 is not fixed in the radial direction of the unison ring 5. Thus, control operations such as dimension measurement are not easy to perform. Accordingly, deterioration in productivity and reliability is inevitable.
According to the disclosure in Japanese Laid-Open Utility Model Publication No. 61-49002, the fit-engagement groove (communication fit-engagement groove) has a substantially semi-circular configuration. However, from the viewpoint of the engagement relationship with respect to the fit-engagement portion of the drive arm, it is to be presumed that the fit-engagement groove has a U-shaped configuration. Thus, also in the technique disclosed in the above-mentioned publication, a problem similar to that of related-art example 1 is involved.
In the variable nozzle mechanism, the fit-engagement portion of the drive arm contacts the closing side surface of the fit-engagement groove of the unison ring. There is a need in the art for a variable nozzle turbocharger in which the contact stress is low and which has high productivity or high reliability.